1. Field of the Invention
The present invention relates to an oscillator whose oscillation frequency is proportional to the intensity of the ambient magnetic field, used for measuring said magnetic field and comprising a magnetic resonance magnetometric probe for detecting a signal at the frequency of said magnetic resonance, in response to a low frequency excitation signal, and means for generating said low frequency excitation signal in response to the detected signal.
The present invention relates then more particularly to nuclear magnetic resonance magnetometer (NMR) probes.
2. Description of the Prior Art
Such probes are known in the technique and different types of NMR magnetometers are described for example in the French patent applications 1 447 226, 2 098 624 and 2 583 487. It will be recalled that this apparatus comprises at least two liquid samples contained in flasks, these flasks being disposed in a resonating cavity excited at very high frequency. A detector comprises windings about said flasks for picking up and re-injecting a frequency at the Larmor frequency depending, on the one hand on the magnetic field in which the probe bathes and, on the other hand, on the natural gyromagnetic ratio of the liquid samples contained in the flasks.
FIG. 1 shows one example of a NMR probe mounted in a nuclear oscillator in accordance with the prior art. In this figure, block 1 designates a NMR probe comprising two flasks 2 and 3, a detector and low frequency excitation winding 4, the whole being disposed in a cavity excited by a very high frequency signal, designated VHF. The middle point of the winding 4 is here connected to ground and the two ends of winding 4 form two accesses to the probe. A double filter 5, for eliminating the VHF frequency, is provided with two inputs connected to these two accesses and two outputs. Between these two outputs are disposed a succession of parallel capacitors 6, only two of which have been shown and which may be brought into service or not by switches 7 for filtering out the noise around the Larmor frequency, in relation with the shelf inductances of winding 4 and resistors 8, resistors 8 being also connected in parallel across the capacitors 6. The outputs of the double low frequency filter 6, 7, 8 thus formed are connected to the inputs of a wide band amplifier circuit 9 comprising a pre-amplifier and amplifier stages. The output of the amplifier circuit 9 is fed, on the one hand, to the middle point of resistors 8, possibly through a phase shift circuit 10 introducing a 90.degree. phase shift and, on the other hand, to a frequency measuring assembly comprising a digital filtering and digitization system 11 and a frequency meter 12 which is generally connected to a high precision clock 13. The output of the frequency meter 12 is for example applied to a microprocessor for controlling the information received which represents the measured values of the magnetic field and possibly coding them and re-emitting them to a user circuit.
Thus, the assembly of elements 1 to 10 forms what is usually called a nuclear oscillator which oscillates normally at the precision frequency of the nuclear spins (or Larmor frequency f.sub.0) if suitable gain and phase conditions are respected. Now, because of geometric and electronic dissymetries or parasite effects on the windings and coils (capacitive effects, coupling between coils), such as circuit risks forming an oscillator at the natural frequency of the coils, generally between 10 and 25 kHz depending on the parasite capacity in parallel with the coils, particularly that due to the cables connecting the coils to the amplifier. It is then necessary to inject an excitation signal free of any energy outside a band of a few hundred Herz about the Larmor frequency, which is generally between 1000 and 3000 Hz depending on the conditions in which the probe is placed. In the conventional circuit shown in FIG. 1, this problem is solved by he above-mentioned filtering assembly formed of resistors 8 and one or more capacitors 6 which are selectively connected in parallel with the coils, depending on the state of switches 7 (in practice, these switches 7 will be semi-conductor switches, e.g. MOS transistors).
This solution has several drawbacks related both to the considerable and variable phase shift caused by this filtration mode and to the temperature sensitivity which results therefrom, without counting the problem raised by the initial choice of the capacitor to be connected in the filter. Such filtering cannot have a very narrow passband. It will at best give passbands of the order of a few hundred Hz.
Furthermore, even if the assembly of elements 1 to 10 of FIG. 1 made it possible to construct a satisfactory nuclear oscillator, there remains the operating frequency of the oscillator to be measured. For that, in the prior art, the clumsy solution shown is generally used which consists in using digitization and digital filtering by filter 11 before making a measurement with a frequency meter. In fact, measurement of the frequency of a signal is conventionally obtained from measurement of the period T.sub.s of this signal by means of a precision clock of frequency F.sub.H whose number N.sub.p of periods is counted during a time corresponding to a multiple of the periods of the signal, nT.sub.s, the measured frequency is then equal to: EQU f=nF.sub.H /N.sub.p.
This procedure can only be used if the signal whose frequency it is desired to measure has a low noise corresponding to an instantaneous relatively low random phase, i.e. less than 2.pi. by at least one order of size. In addition, in order to obtain high accuracy, the signal must be filtered so as to eliminate the noise energy outside the band .+-.1/2T.sub.c about the resonance spectral line. Tc being the counting time (T.sub.c =nT.sub.s). Thus, the well known phenomenon of frequency folding in the presence of sampling is minimized and the frequency measurement is thus obtained with the required accuracy. But this results in the need, in prior art techniques, to use a digital filter before the frequemcy meter. It will be noted that such a digital filter forms a clumsy apparatus itself comprising microprocessors.
Furthermore, even with those precautions, the measured frequency will not be in the Larmor frequency f.sub.0 but a frequency f.sub.m =f.sub.0 +tan .phi./T, where T is the relaxation time of the probe which varies with the temperature and o a phase shift related more particularly to the presence of the switchable capacity filter and to the different amplifier stages. This phase shift cannot be predetermined and corresponds then to an error. In fact, the error factor tan .phi./T is of the order of 10.sup.-3 f.sub.0. This does not a priori form a limitation for the conventional measurements. In fact, it is not generally f.sub.0 that it is desired to measure but the variations of f.sub.0 and if the error term were constant that would not raise any problem. But in fact, it is apparent that the values T and .phi. vary with time and the temperature and that thus, basically, this system cannot provide an accuracy greater than 10.sup.-6 over periods longer than a minute.
Consequently, an aim of the present invention is to provide a new nuclear oscillator circuit minimizing the drawbacks of the above mentioned prior art devices.
A more particular aim of the present invention is to filter simply the oscillation signal of a nuclear oscillator with a small-sized circuit, of low consumption and low cost.
Another aim of the invention is to filter the nuclear signal re-injected into the nuclear oscillator so as to avoid any parasite oscillation at the natural frequency of the coils.
Another aim of the present invention is to provide such a system which makes possible a frequency measurement with an accuracy of the order of 10.sup.-8.
Another aim of the present invention is to combine the function of filtering the signal preceding the frequency meter with the function of filtering the reinjected nuclear signal, in a single device placed inside the nuclear oscillator.
In other words, a problem which the invention aims at solving is the correct operation of a nuclear oscillator throughout the whole range of possible operating frequencies (currently from 1000 to 3000 Hz) and in a vast range of temperatures.
Another problem which the present invention aims at solving concerns the measurement of the frequency with an accuracy of some 10.sup.-8 for frequencies going from 1000 to 3000 Hz.